Hi Dermot, I would say 1.5 gives to much back pressure. I always come straight out of the pump with 2" pipework, Depending on how much water You are moving 2" right through will be fine, but if You have a 15,000 or 18,000 pump I would say have a 3" spray bar. I usually use an 8mm drill bit and fit as many equally spaced holes as I can in three rows. One center line and then another line of holes 45% either side of the original center line. I like to see the water pouring out of the holes rather than spraying as this to be indicates back pressure. Try to keep pipework as straight as possible and rather than use a 90% try to use either two 45% or a swept bend.Hope I have been of some help.Justin